This study will demonstrate the effects of long-term ingestion of high fat diets on serum prolactin and on induced and spontaneous mammary tumors in rats. The relationship between fatty acid composition and hormonal and tumorigenic effects will be defined. Evideence that ingestion of a high fat diet enhances rat mammary tumorigenesis will be assessed, employing DMBA-induced tumor incidence in rats fed one of several dietary fats. Natural dietary fats of widely varying fatty acid composition will be fed as a purified diet, serum prolactin will be monitored by RIA, and DMBA-induced and spontaneous mammary neoplasms idenfified. The possibility of predicting ultimate tumor response of the individual from serum prolactin profiles will be determined by comparison of periodic serum prolactin paired values (proestrus-estrus: metestrus-diestruso with tumor development. The distribution of prolactin receptors in various mammary lesions will be determined by autoradiographic procedures. Dietary fat effects on the pathogenesis and incidence of spontaneous or induced benign or malignant mammary tumors will be differentiated.